1. Field of the Invention
The present invention relates to active phased array antennas; and more particularly, to a system and method for controlling transmit/receive modules of active aperture phased array antennas.
2. Discussion of Related Art
Active aperture phased array antennas have a multitude of radiating elements that alternately transmit RF energy and receive return energy from a target or terrain. Typically, each one of the radiating elements or apertures has an associated transmit/receive module (hereinafter referred to as a T/R module) that prepares the antenna system for each alternate transmit/receive cycle. Each T/R module typically includes a transmitter amplifier that is activated near the beginning of each transmit cycle for amplifying the RF energy transmitted during such cycle, and deactivated at the beginning of each receive cycle for permitting the elements to receive the return energy or echoes. Each T/R module also includes a receiver protector and a receiver amplifier. The receiver protector of each T/R module is activated at the beginning of each transmit cycle to prevent the transmitting RF energy from damaging the receiver, and deactivated during the receive cycle for permitting the receiver to receive the return energy. The receiver amplifier of each T/R module is deactivated at the beginning of each transmit cycle to permit the transmission of the RF energy; and activated at the beginning of each receive cycle to amplify the received return energy. An RF pulse is transmitted during each transmit cycle while the receiver amplifier as in a deactivated condition.
Typically, a single T/R signal is transmitted simultaneously to all of the T/R modules of an antenna for switching "on" or activating, and switching "off" or deactivating the transmit amplifiers, receive amplifiers, and the receiver protectors. When the T/R signal changes from a low to a high state, it marks the beginning of a receive cycle, and when it changes from a high to a low state, it marks the beginning of a transmit cycle. It is important that all the transmit amplifiers, receiver amplifiers, and the receiver protectors of all of the T/R modules of the array turn on and off simultaneously at the appropriate times during the respective transmit/receive cycles. If the different T/R modules acted at different times, the antenna would not operate as a coherent whole.
The relative timing for the activation and deactivation of the amplifiers and protectors within each individual T/R module is also important. For example, if both the receive and transmit amplifiers should be active at the same time, the antenna would be subject to oscillations. Also, if the receiver protector is off while the transmit amplifier is active, the receiver amplifier would be damaged. Even if the deviation in optimum timing produces no oscillation or component damage, system performance degradation would result. Therefore, it is important to system performance and integrity, that the activation and deactivation of the amplifier and protector elements occur at the proper time.
Although, all of the T/R modules respond to a single T/R signal, there is a significant degree of variation between T/R modules in the amount of time required to switch on and off; or in other words, activate and deactivate, the transmit and receive amplifiers, and the receiver protectors. For example, there is a great deal of variation in the response of circuit elements; that is, some power transistors switch faster than others. Also, there are variations in delays through the distribution electronics that supply the T/R signal to the T/R modules; and of course there are varying delays because of variations in wafer fabrication processes of the digital electronics that control the RF elements.
In actual practice, at the beginning of each transmit cycle, the receiver protector should be activated at the same time that the receiver amplifiers are deactivated, and certainly before the transmit amplifiers are turned on, or activated. At the beginning of each receive cycle, the transmit amplifiers should be first turned off, the receiver amplifiers activated, and the receiver protectors deactivated at a set time interval later.
It has been determined that to maximize system performance, all switching should occur within 12.5 nanoseconds of the specified time; and the required time limit for switching between a transmit and a receive cycle is one hundred nanoseconds. It has been proposed to utilize a controller regulator interface for each T/R module that makes sure the T/R module receiver amplifier is off, before it will permit the transmit amplifier to be activated, or powered up. The proposed controller includes a feedback monitor that makes sure the transmit amplifier is deactivated, or powered down, before it permits the T/R module to turn on the transistor, or switch, that supplies power to the receiver amplifier. The receiver protector is intended to be deactivated any time the receiver amplifier is activated or powered up. Although suitable for the purposes intended, this approach makes it difficult to meet the stringent system requirements because of the delays involved in monitoring the receiver amplifier before enabling the transmit amplifier. This problem also exists in switching from the receive cycle to the transmit cycle. Such an approach is also subject to variations in circuit elements as heretofore mentioned.